1. Field of the Invention
The present invention relates generally to oilfield operations and more particularly methods and apparatus using fiber optics in coiled tubing operations in a wellbore.
2. Description of Related Art
Casing collar locator (CCL) tools, resistivity tools, and spinner tools are known in the oilfield industry and are used commonly in wireline applications. The use of coiled tubing as a different type of wellbore conveyance in wellbore applications is increasing, resulting in a need for downhole apparatus and methods adapted for use with coiled tubing. Difficulties inherent with using downhole electromechanical apparatus with coiled tubing are the lack of power to the downhole apparatus and the lack of telemetry from the downhole apparatus to the surface; both of these functions are performed by wireline in conventional wellbore applications. To address these difficulties, it is known to install electrical wireline in coiled tubing. Although adding wireline to coiled tubing operations increases the functionality of the coiled tubing, it also increases the cost of the coiled tubing string and complicates field operations. The addition of wireline to a coiled tubing string significantly increases the weight of a coiled tubing string. Installation of the wireline into the coiled tubing string is difficult and the wireline is prone to bunch into a knotty mass or “bird nest” within the coiled tubing. This, and the relatively large outer diameter of wireline compared to the internal diameter of coiled tubing, can undesirably obstruct the flow of fluids through the coiled tubing, such flow through the coiled tubing frequently being an integral part of the wellbore operation.
It is also known to use fiber optics to make downhole measurements by providing optical power at the surface to the fiber optics and using that optical power to generate motive power in a wellbore. For example, U.S. Pat. No. 6,531,694, incorporated herein by reference, discloses a fiber optic system comprises an optical power source at the surface and a fiber optic loop from the surface down the wellbore and back up the wellbore. The optical power from the surface light source is disclosed to power a downhole light cell, which in turn generates electricity to trickle charge batteries in the wellbore. Similar to power being sent downhole, measurements and borehole information may be conveyed to the surface via the fiber optic system. What is not disclosed, however, is the using the measurement of downhole elements to generate energy to send measurements or information to the surface via fiber optics.
Others have attempted to generate power downhole instead of relying on a power source at the surface. It is known to use batteries downhole for power; for example, one existing tool uses six to twelve feet of batteries. Such configurations are accompanied by operational constraints and difficulties. What is needed is a system and method for making downhole measurements with coiled tubing, and communicating those measurements to recording devices on the surface, but without an extensive external power source for the downhole measuring equipment, and without the weight of electrical wireline. Furthermore, what is needed is a device that uses sufficiently small amounts of supplemental power, that such power can be supplied by small batteries that would extend the length of the tool by as little as two inches.